Protective covering for bonded wire electric strain gages



D. MCHENRY July 15, 1947.

PROTECTIVE COVERING FOR BQNDEDWIRE ELECTRIC STRAIN GAGES Filed Feb. 28, 1 946 mmvroze. v UOUGL/LS Mafia/war Arm/PMS) Patented July 15, 1947 ELECTRIC STRAIN .GAGES Douglas Mcllenry, Denver, 0010., assignor to United States of America, as represented by the Secretary of the Interior Application February 28, 1948, Serial No. 650,938

2 Claims. (Cl. 201-88) (Granted under the act oi March 3, 1883, as amended April 80, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by the Government of the United States for governmental purposes, without the payment to me of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (ch. 460, 45 Stat. L 467).

My invention relates to a moisture-proof covering for bonded wire electric strain gages and particularly to a vulcanized rubber button for exing one of my buttons to a steel reinforcing bar when positioned over and operatively connected with a strain gage.-

In these drawings, ilindicates'a bonded-wire electric strain sensitive gage comprising a ilne wire filament which is adhesively bonded to paper by means of Celluloid resin cement or the like while in an initially untaut condition. The gage ii is shown as adhesively fastened to a specimen ii of a material to be tested, which specimen may be concrete with or without reinforcement. The gage I l is coveredwith a layer I! of felt or the like. A conductor cable It enclosing two lead wires i5, it has been threaded through a vulcanized rubber button ll having a suitable recess II for the strain gaze and the felt covering therefor for avoiding direct contact of my button and the sage, and a hole I9 for the cable It. A margin 20 of the button ll forms a frame around the recess ll of a width adequate for securely attaching the button I! to a speci- In Figure 3 is shown one-half II of a two-part mold which is bolted by bolts 22 and is used for making my button I l of neoprene" or other like vulcanizable compound, and is also used for vulcanizing the said button to a bar .2! of steel or other material.

The method of attaching my buttons to sampies to be strain tested will vary with the particular requirements of the test. In the embodiment illustrated and described herein my button may be simply attached to a specimen by a waterproof elastic cement without heat or pressure and this elatsic cement will also seal the cable H in the button i1.

'When used for measuring the strains in reinforcing steel, where the rubber-metal Joint must withstand the rough treatment involved in concrete placing operations, a remarkably strong Joint may be made by applying vulcanizlng cement to the steel specimen, overlaying this with a thin layer of semi-cured "neoprene and vulcanizing the joint under pressure by means of a mold 2| in which my buttons are made. I

Another method of attaching the gage with my moisture-excluding button IT to a reinforcing metal bar comprises grounding to the bar one of the two lead wires of the gage by inserting it into a an: inch drill and there securing it by means of a wedge or by soldering. The strain gage is then cemented to the bar and allowed to dry.. I1 the gage lacks an attached layer or protective felt, a piece of felt is trimmed to fit the gage and cemented thereto. The other insulated terminal lead wire is doubled back across this felt layer and is secured in this position by a second layer 01 felt cemented over it. The insulated cable It is then wrapped in a thin layer of semi-cured neoprene" and inserted in the hole i8, bending the bare end of the wire therein to lie flat against the top of the cavity, and soldering it to a lead wire of the ga e. The button is then either cemented 0r vulcanized to the specimen to form a moisture-proof Joint.

My rubber housing provides a means for maintaining the required accuracy or the electrical resistance measurements with strain gages by keeping these gages completely dry during use, even in the interior of concrete under water.

I have shown and described one embodiment of my invention, but it is to be understood that it is capable of many modifications. Changes, there- WhatIclaim is: strain gage against a surface of a specimen to 1. A protective button for an electric strain V be tested. gage having a chamber to receive a strain case DOUGLAS McHENRY. and a passage connecting with said chamber to receive conductors to a strain gage, said button 5 REFERENCES CITED being of water-proof material and sealed to a The following references are of record in the specimen tobetested. file of this patent 2. A method of attaching a protective covering for strain sensitive gases which includes the UNITED STATES PATENTS- steps of forming a rubber button having a cen- 10 Number Name Date tral opening to receive a terminal, applying a 2,036,458 Carlson Apr. 7, 1936 suitable adhesive to the contact face of the but- 2,350,073 Simmons May 30, 1944 ton and pressing said contact surface over a 2,360,886 Osterber Oct. 24, 1944 

